This invention relates to inrush current limiters for tungsten incandescent lamp filaments, and particularly to an arrangement to efficiently eliminate current overshoot.
It has long been known that the large inrush currents typical of tungsten lamp filaments are detrimental and many suggestions of various negative temperature coefficient thermistors have been made. The large inrush currents have detrimental effects both on the electricl systems supplying the power and on the lamps themselves. A number of detrimental effects on the lamp itself are known. These effects include, for example, the electromechanical effects of the large currents, but the most straightforward effect is probably the failure of the filament due to localized melting caused by local thermal overshoots. In this failure mode, a localized high resistance section is much more intensely heated due to its initial higher resistance. This is then compounded due to the localized resistance rising more rapidly than the rest of the filament (tungsten having a positive temperature coefficient, any local section having greater cold resistance will receive proportionally greater power input, heating that section more than average, causing its resistance to become even higher and thus even more intensely heated). Because the localized section has heated up much faster than the remainder of the filament, it can reach or exceed operating temperature before the total filament resistance has risen to the operation value. Thus, the local section is subjected to higher than steady state operating current even after the local section resistance has risen to or above its steady state operating value. This combination produces intense local heating and temperatures even higher than the steady state temperature of that section. If a localized section reaches the melting point of tungsten, the filament fails.
Despite the long felt need, the thermistor configurations of the prior art have either failed to eliminate the inrush current overshoot or were excessively inefficient. Often the devices were both inefficient and ineffective. Several of the prior art devices, especially those with a very large thermistor mass or with a switching characteristic, delayed the current overshoot but did not eliminate it. While some of the configurations did extend the life of a lamp, this was primarily due to an excessive residual resistance in series with the filament which lowers the voltage across the filament which results in a very inefficient lamp with dramatically lower light output.